1. Field of the Invention
The present invention is directed to self-crosslinking shape memory polymers, their production and use. Particularly the current invention comprises a reaction product of a polymer with a self-crosslinking functional group which will self-crosslink under a given environmental stimulus. More particularly, the current invention comprises a reaction product of a small portion of silane crosslinker which is incorporated into the polymer chain by copolymerizing it with acrylates forming alkylsiloxyl groups in the acrylate copolymers.
Shape memory polymers (SMPs) are a unique class of polymers, which soften and harden quickly and repetitively on demand. This feature provides the ability to temporarily soften, change shape, and harden back to a solid structural state in various new highly detailed shapes and forms. Typical SMPs have a very narrow temperature span in which they transition between hard and soft states. This narrow glass transition temperature span is a key physical property that allows a SMP to maintain full structural rigidity up to a specifically designed activation temperature. Yet with as little as 5° C. to 10° C. increase above that temperature it quickly softens and allows shape change and subsequent re-hardening into new shapes.
2. Background of Prior Art
Shape memory materials are materials capable of distortion above their glass transition temperatures (Tgs), which store such distortion at temperatures below their Tg as potential mechanical energy in the material, and release this energy when heated again to above the Tg, returning to their original “memory” shape. In essence, these materials can be “fixed” to a temporary shape under specific conditions of temperature and stress and later, under thermal, electrical, or other environmental command, the associated elastic deformation can be completely or substantially relaxed to the original, stress-free, condition.
SMAs
The first materials known to have these properties were shape memory metal alloys (SMAs), including TiNi (Nitinol), CuZnAl, and FeNiAl alloys. The shape-memory capabilities of the these metallic materials capable of exhibiting shape-memory characteristics occur as the result of the metallic alloy undergoing a reversible crystalline phase transformation from one crystalline state to another crystalline state with a change in temperature and/or external stress. With a temperature change of as little as about 10° C., these alloys can exert a stress as large as 415 MPa when applied against a resistance to changing its shape from its deformed state. Such alloys have been used for such applications as intelligent materials and biomedical devices. These materials have been proposed for various uses, including vascular stents, medical guide wires, orthodontic wires, vibration dampers, pipe couplings, electrical connectors, thermostats, actuators, eyeglass frames, and brassiere underwires. However, these materials have not yet been widely used, in large part because they are very expensive. Additionally, their applications have been limited due to limited ability to withstand strains greater than approximately 8%.
SMPs
Shape memory polymers (SMPs) are being developed to replace or augment the use of SMAs, in part because the polymers are lightweight, high in shape recovery ability, easy to manipulate, and economical as compared with SMAs. SMPs are materials capable of distortion above their glass transition temperature (Tg), storing such distortion at temperatures below their Tg as potential mechanical energy, via elastic deformation, in the polymer, and release this energy when heated to temperatures above their Tg, returning to their original memory shape. When the polymer is heated to near its transition state it becomes soft and malleable and can be more easily deformed. When the temperature is decreased below its Tg, the deformed shape is fixed by the higher rigidity of the material at a lower temperature while, at the same time, the mechanical energy expended on the material during deformation will be stored. Thus, favorable properties for SMPs will closely link to the network architecture and to the sharpness of the transition separating the rigid and rubbery states.
Polymers intrinsically show shape memory effects on the basis of rubber elasticity, but with varied characteristics of temporary shape fixing, strain recovery rate, work capability during recovery, and retracted state stability. The first shape memory polymer (SMP) reported as such was cross-linked polyethylene; however, the mechanism of strain recovery for this material was immediately found to be far different from that of the shape memory alloys. When the polymer is heated to a soft, pliable state, it can be deformed under resistance of ˜1 MPa modulus. When the temperature is decreased below the glass transition temperature (Tg), the deformed shape is fixed by the higher rigidity of the material at lower temperature while, at the same time, the mechanical energy expended on the material during deformation will be stored. When the temperature is raised above the Tg, the polymer will recover to its original form as driven by the restoration of network chain conformation entropy. Thus favorable properties for SMPs will be closely linked to the network architecture and to the sharpness of the transition separating the rigid and rubber states. Compared with SMAs, SMPs can withstand high strains, typically at least 100% 400%, while the maximum strain of the SMA is typically less than 8%. As an additional advantage, due to the versatility of polymers, the properties of SMP can be tailored according to the application requirements, a factor that is very important in industry.
Several physical properties of SMPs other than the ability to memorize shape are significantly altered in response to external changes in temperature and stress. These properties include the elastic modulus, hardness, flexibility, vapor permeability, damping, index of refraction, and dielectric constant. The elastic modulus (the ratio of the stress in a body to the corresponding strain) of an SMP can change by a factor of up to 200 when heated above its melting point or glass transition temperature. Also, the hardness of the material changes dramatically when it is at or above its melting point or glass transition temperature. When the material is heated to a temperature above the melting point or glass transition temperature, the damping ability can be up to five times higher than a conventional rubber product. The material can readily recover to its original molded shape following numerous thermal cycles.
Heretofore, numerous polymers have been found to have particularly attractive shape memory effects, most notably acrylates, polyurethanes, polynorbornene, styrene-butadiene copolymers, and cross-linked polyethylene.
In the literature, polyurethane-type SMPs have generally been characterized as phase segregated linear block copolymers having a hard segment and a soft segment. The hard segment is typically crystalline, with a defined melting point, and the soft segment is typically amorphous, with a defined glass transition temperature. In some embodiments, however, the hard segment is amorphous and has a glass transition temperature rather than a melting point. In other embodiments, the soft segment is crystalline and has a melting point rather than a glass transition temperature. The melting point or glass transition temperature of the soft segment is substantially less than the melting point or glass transition of the hard segment.
Examples of polymers used to prepare hard and soft segments of known SMPs include various polyethers, polyacrylates, polyamides, polysiloxanes, polyurethanes, polyether amides, polyurethane/ureas, polyether esters, and urethane/butadiene copolymers. See, for example, U.S. Pat. No. 5,506,300 to Ward et al.; U.S. Pat. No. 5,145,935 to Hayashi; U.S. Pat. No. 5,665,822 to Bider et al.; and U.S. Pat. No. 6,720,420 to Langer et al.
Conventional SMPs generally are segmented polyurethanes and have hard segments that include aromatic moieties. U.S. Pat. No. 5,145,935 to Hayashi, for example, discloses a shape memory polyurethane elastomer molded article formed from a polyurethane elastomer polymerized from of a difunctional diisocyanate, a difunctional polyol, and a difunctional chain extender.
Recently, however, SMPs have been created using reactions of different polymers to eliminate the need for a hard and soft segment, creating instead, a single continuous piece of SMP. U.S. Pat. No. 6,759,481 to Tong, discloses such a SMP using a reaction of styrene, a vinyl compound, a multifunctional cross-linking agent and an initiator to create a styrene based SMP.
Waterborne Polymers
Waterborne polymer dispersions are rapidly becoming the coating of choice for an increasing number of industrial and applications, thanks to their being environmentally, healthy and relatively safe. Indeed, over the last 20 years a rapid improvement of both performance and production costs has been prompted by a better understanding of the chemistry and mechanisms of film formation and development of main microscopic properties relevant to coating applications, such as adhesion, cohesion, curing mechanisms, surface properties, surface dynamics, and stability against aging.
However, while the technology gap between conventional solvent-based and waterborne coating formulations has progressively narrowed, some issues related to mechanism and physics of the film formation and to the role of the various components in providing a given coating performance are still challenging and require further research.
Among the features to be considered as a selection of functional groups or additives inducing the cross-linking in a latex polymer, the nature of the chemical reaction involved and schematics are crucial. In fact, untimely cross-linking during polymerization or in the latex dispersion can negatively affect both colloidal stability, causing coagulation, and film formation, hampering the last stage of inter-particle polymer diffusion.
Despite the many excellent properties of siloxanes, the poor compatibility between poly-siloxane and acrylics (or acrylic styrene) brings disadvantages to their polymerization and products, so trialkoxysilane has often been used as a cross-linking agent to form a middle transition later between the core and show to improve the compatibility between poly-siloxane and poly acrylics. Much research on the cross-linking poly-siloxane products and on improving product properties have been reported; however, reports about the self cross-linking properties of organo-silicone acrylic emulsions with vinyl trialkoxysilane have seldom been seen. One report states that latex particles incorporate these chemicals to give them self cross-linking properties in the film forming process. However, this report shows that the film is cast and then cross-linking occurs additionally only between latex to latex particles but not spontaneously within the polymer composition. Therefore there is still a need in the industry for a film that is cross-linked during casting instead of after and which cross-links across the entire mixture not just one portion to another.